The present invention relates to circuitry for switching a microcomputer to a standby mode once the microcomputer is in a defined state, normally a reset state.
It is known that, if one wishes to retain information in a random access memory (RAM), it is necessary to provide a standby power supply connected to the RAM even when an associated microprocessor is switched off. Care must be taken, however, to ensure that the information contained in the RAM is not changed when the microcomputer is switched off in the middle of accessing a RAM.
Various circuits have been proposed for achieving a controlled switch-over to standby supply for a RAM. Unfortunately, many of these circuits utilize a number of components and occupy a relatively large amount of space. Until now, this has rendered them inappropriate for incorporation in hybrid devices used for multicircuit electronic equipment used in motor vehicles.
U.S. Pat. No. 4,512,019 describes a circuit for monitoring a microcomputer and, in particular, a circuit for resetting the microcomputer when a malfunction occurs or when the supply voltage drops to a value below a predetermined level.
In Patent Abstracts of Japan, Vol. 9, No. 214, p. 384 and JP 60-73 720 a device for preventing the destruction of stored data in a RAM is described. In this device the base of a switching transistor installed between the main power source and the power input terminal of the RAM is connected to the output terminal of a voltage-drop detecting element.
A device for maintaining operation as long as the voltage fluctuations of a main power source are within a predetermined range of recommended working source voltages of a CMOS-RAM is described in Patent Abstracts of Japan, Vol. 10, No. 389, P. 531 & JP 61-177 523. Therefore a relay contact for switching between the main power source and a chargeable type secondary backup battery is proposed.